As known, low voltage switching devices are used in electric circuits or grids to allow the correct operation of specific parts of these latter.
For instance, low voltage switching devices ensure the availability of the nominal current necessary for several utilities, enable the proper insertion and disconnection of electric loads, protect (especially circuit breakers) the electric grid and the electric loads installed therein against fault events such as overloads and short circuits.
Numerous industrial solutions for the aforementioned switching devices are available on the market.
Conventional electro-mechanical switching devices generally have an external case that houses one or more electric poles.
Each pole comprises a couple of separable contacts to break and conduct current.
A driving mechanism causes the movable contacts to move between a first closed position, in which they are coupled to the corresponding fixed contacts, and a second open position, in which they are spaced away from the corresponding fixed contacts.
In closed position, well designed contacts result in quite low power losses, whereas in open position they provide a galvanic (electrical) isolation between the portions of the electric poles that are electrically upstream and downstream connected, provided that their mutual physical separation is above a minimum value.
Such a galvanic isolation is very important in common practice, since it enables safe repairing and maintenance works on the circuit in which the switching device is inserted.
Although such conventional switching devices have proven to be very robust and reliable, in direct current (“DC”) applications, and mainly at relatively high voltages (up to 1500V), the interruption time can be quite long, and therefore electric arcs, which usually strike between mechanical contacts under separation, may consequently last for a relatively long time.
Severe wear of the contact may thus arise, with a consequent remarkable reduction of the electrical endurance, i.e. the number of switching operations that a switching device can perform.
In order to face with such issues, so-called Solid-State Circuit Breakers (“SSCBs”) have been designed, which adopt, for each electric pole, one or more solid state switches for current breaking purposes.
Typically, solid state switches are semiconductor-based switching devices that can commutate between an on-state and an off-state.
The main advantage of SSCBs resides in that they have potentially unlimited electrical endurance due to their arcless breaking operations.
Further, their interruption time is remarkably shorter in comparison with the interruption time of the electro-mechanical switching devices.
On the other hand, SSCBs generally require intensive cooling to remove the heat generated by the current flowing through the solid state switches, when these latter are in an on-state.
An even more relevant drawback resides in that SSCBs are not suitable for providing a galvanic isolation between upstream and downstream connected portions of the electric poles.
In fact, small currents (leakage currents) flow through the solid state switches, even if these latter are in an off-state.
In order to mitigate these problems, there have been developed hybrid solutions, in which, for each electric pole, conventional electro-mechanical switches are electrically connected in parallel and/or in series with the solid state switches of the pole.
Hybrid SSCBs have proven to be quite reliable and effective in their operation but they are affected by some drawbacks, too.
Generally, they are relatively bulky and difficult to install on the field. Further, they have a relatively complex constructive layout that is often expensive to realize at industrial level.
In addition, the operations of the solid state and electro-mechanical switches must be managed according to very precise time sequences and a tight timing.
Therefore, in the market it is still felt the demand for technical solutions capable of solving, at least partially, the drawbacks mentioned above.